Ultrahigh vacuum sublimation pump

ABSTRACT

An evacuable chamber includes an upper bell jar portion, which serves as a working space, and in communication therewith a lower pump portion having the shape of a rigid bellows. The bellows is submersed in a refrigerant, such as liquid nitrogen, and surrounds a suitable source of a sublimable getter material, such as titanium-molybdenum filament. The upper portion illustratively has a pair of apertures, one of which communicates with a liquid helium cold trap and the other of which communicates with a roughing pump. In a preferred embodiment the roughing pump comprises in combination a molecular sieve adsorption pump and a cold finger submersible in liquid helium. During operation, the filament is heated above its sublimation temperature to form a thin film of titanium on the interior surface of the bellows. When cooled, the thin film of titanium effectively adsorbs all gases except He and Ne and will additionally adsorb light hydrocarbons, such as methane, if the film is maintained at a temperature below 180 degrees K. The cold trap pumps both He and Ne. The system as a whole is effective not only in producing chamber pressures of at least 10 11 Torr in less than ten hours, depending on the volume of the chamber, but also in maintaining that pressure under working conditions at dynamic pumping speeds in the millisecond range.

` 1111 3,8MH94 1451 May 21, 1974 United States Patent 1191 Dunkleberger et al.

[54] ULTRAHIGH VACUUM SUBLIMATION Primary Examiner-William L. Freeh PUMP Attorney,

Agent or Firm-M. J. Urbano Inventors: La Rue Norman Dunkleberger,

[57] ABSTRACT An evacuable chamber includes an vupper bell jarA por- Berkeley Heights; Lawrence Yun Lung Shen, Summit, both of NJ.

Assignee: Bell Telephone Laboratories ti which serves as a working space, and in communication therewith a lower vpump portion having the shape of a rigid bellows. The bellows is submersed in a refrigerant, such as liquid nitro Incorporated, Murray Hill, Berkeley Heights, N.J.

aL. s 1 h mmm Sn.. mum www maw amu )r nem T [22] Filed: Nov. 22, 1972 suitable source of a sublimable g titanium-molybdenum lustratively has a pair of municates with a li Apptv NQ.; 308,665

apertures, one of which comquid helium coldV trap andthe other FMbS/g icates with a roughing pump In a 417/49" of which commun preferred embodiment the roughing pump comprises 389 417/49 Y in combination ption pump quid helium. Durin operation, the filament is heated above its sublimatio [58] riem of Search 55/208 a molecular sieve adsor [56] References Cited and a cold finger submersible in li UNITED STATES PATENTS temperature to form a thin film of titanium on the interior surface of the bellows. When cooled film of titanium effectively adsorbs all the thin enaDibt Hat ecm Ica SO wmd .IS de ee C I Xyn Or h.l U e a hS Stm WS ehi e Sga af 3.1 D: glm .r S e .l e im am VJC g and Neand will additionally adsorb bons, such as methane, if the f1 temperature below 180 degrees K. The cold tra 417/49 pumps both He and Ne. The s fective not only in producin least l011 Torr in less than ten hours, depending on the volume of the Chamb that pressure under working conditions at d er, but also in maintaining ynamic pumping speeds in the millisecond range.

Germany 417/49 l 30 Claims, 1 Drawing Figure COLD TRAP 300 j' cRYoGENIc RouGHtNe PuMPgpg PATENTEUMMel i974 n1! mmm mism@ .jmsoc IW *LIN N i5@ 53214 .www Q mia om @om vom ULTRAHIGI-I VACUUM SUBLIIVIATION PUMP BACKGROUND OF4 THE INVENTION This invention relates to ultrahigh vacuum pumps and more particularly to relatively high dynamic speed, low cost sublimation pumps capable of producing, and maintaining under working conditions, subatmospheric pressures at least as low a 10l1 Torr..

The maintenance of an ultrahigh vacuum (uhv) under dynamic conditions depends largely on the. pumping speed of the system at the operating pressure. As an example, to purify niobium the pressure is maintained around 109 Torr with the impure niobium sample heated to about 2,500 degrees C, thereby generat ing an appreciable gas load. In order to reach pressures in the l*l1 Torr range, an all stainless steel chamber utilizing copper gasket seals is typically used in the prior art. Commercially available prior art systems generally cost more than $20,000. and are very susceptible to contamination. The high cost of present commerical uhv systems is due mainly to the use of a massive magnetic ion pump and large demountable seals, the latter being necessary to provide high conductance between the chamber and the ion pump. v

A second disadvantage of prior art systems is that a typical 500 liter/sec ion pump and steel bell jar together weigh about 1100 lb. Such a large mass of steel requires very large heating powers to effect outgassing at about 200 degrees C within a reasonably short time. Moreover, a relatively long time is generally required to reach operating pressures in the lo to 10*l1 Torr range starting from an initial chamber pressure of one atmosphere. In addition, constant loading of gases into an ion pump continuously decreases its pumping speed 3 and lowers its performance. As a result, such prior art vacuum systems are not suitable for frequent cycles between one atmosphere and very low pressure or for operation with heavy gas loads. In fact, because the cost to regenerate a dirty ion pump is about one third the price of a new pump, a high conductance metal valve is ordinarily used to prevent the ion pump from being exposed to air. Unfortunately, such a valve is often more expensive than the ion pump itself.

There is therefore a need for a relatively lightweight, low cost uhv pump which, starting from one atmosphere, is capable of reaching pressures of l0'l1 Torr in reasonably short times (e.g., 10 hours) and which furthermore is capable of maintaining such pressures under dynamic loading (i.e., working) conditions. As mentioned above, prior art ion pumps are extremely costly, and often' inadequate in their dynamic pumping performance. In addition, such pumps are undesirable in many environments which for technological reasons should be free of magnetic fields. Thus, the large permanent magnet of the ion pump generates a magnetic field which would interfere, for example, with electron diffraction apparatus being used in conjunction with the vacuum chamber. Although a variety of magnetic pole piece arrangements have been fabricated in the prior art in an attempt to provide a magnetic-field-free environment, it is nevertheless difficult to compensate the residual field of the pump magnet unless the ion pump is placed far away from the working chamber. However, to so place the pump decreases the pumping speed by reducing the conductance between the chamber and the pump.

Consequently, there is also a need for a uhv pump capable of producing desired low pressures and a high pumping speeds at low cost without generating undesired magnetic fields in the working environment.

Finally, the ion pump presents a serious drawback to the attainment of a truly clean vacuum system. That is, when the high voltage of an ion pump is turned off, the main backstreaming product into the chamber is methane gas. Consequently, any time the ion pump voltage is turned off the vacuum chamber is contaminated. It should be further noted that although such ion pumps are commercially offered for sale in conjunction with a titanium sublimation pump, the latter has been advertised as being incapable of pumping methane.

Thus, there is also need for a uhv pump which itself does not introduce contaminants into the vacuum chamber and which is capable of pumping methane gas.

SUMMARY OF THE INVENTION Accordingly, our invention illustratively comprises, in combination, an evacuable chamber including an upper bell jar portion, which serves as a working space, and in communication therewith a lower sublimation pump portion. The lower portion is submersed in a refrigerant, such as liquid nitrogen, and surrounds a suitable source of a sublimable getter material such as a titanium-molybdenum filament. The upper portion illustratively has a pair of apertures, one of which is in gas flow communication with a liquid helium cold trap and the other of which is connectable in gas flow communication with a roughing pump. In a preferred ernbodiment the roughing pump comprises a liquidnitrogen-cooled molecular sieve adsorption pump in combination with a cold fingersubmersible in a liquid nitrogen-liquid helium double dewar, both of which are connectable, through appropriate valves and a common tube, with the other one of the apertures.

In operation, the chamber is first roughed out; that is, its pressure is reduced to about 106 Torr by sequentially opening appropriate valves to the molecular sieve and then to the cold finger and by heating the chamber to about degrees C. Next, the filament is heated above its sublimation temperature to form a thin film of titanium on the interior surface of the lower pump portion. The latter advantageously has the shape of a bellows in order to increase4 the surface-to-volume ratio of the sublimation pump and thereby to increase pumping speed for a given volume sublimation pump. When cooled, the titanium film effectively adsorbs all gases, except He and Ne, by forming stable compounds on the surface of the lower pump portion. Additionally, we have discovered that the titanium film effectively adsorbs light hydrocarbons, such as methane, if the film ismaintained at a temperature below about degrees K. In the foregoing embodiment this result is achieved by submersing the bellows in liquid nitrogen which is at a temperature of about 77 degrees K.

The attainment of chamber pressures of 10ll Torr is achieved not only by the cooperation of the unique roughing pump and the cooled titanium sublimation pump, but also by means of the cold trap which pumps both He and Ne as well as Ar. The latter gas, however, is also pumped by the titanium lm if the film is maintained at a temperature below about 120 degrees K. Advantageously, neither the roughing pump nor the cold trap generates any magnetic fields, thereby permitting the chamber to be utilized for operations which require a field-free environment.

Illustratively, starting from a pressure of one atmosphere, the system as a whole is effective to reduce the pressure of a 60-liter chamber to at least l0l1 Torr in about eight hours. Moreover, under simulated working conditions, gases intentionally introduced into the chamber were removed in a matter of milliseconds, demonstrating a very high dynamic pumping speed. Under such conditions, we found that our pump is capable of reproducing ultimate pressures of ll1 Torr even after more than 60 cycles to atmospheric pressure. Yet, our system weighs about ten times less than prior art ion pumps (e.g., about 150 lb) and costs about l0 times less (e.g., about $2,000.),

One feature of our invention is its ability to increase the purity of commercially available, 3 9s pure niobium, tol 6 9s pure niobium which is not commercially available. It is expected that such high purity niobium, when used as a getter in place of titanium, will further improve the performance of our pump.

BRIEF DESCRIPTION OF THE DRAWING Our invention, together with its various features and advantages, can be easily understood from the following more detailed description taken in conjunction with the accompanying drawing in which the sole FIGURE is a partially cut away view of an illustrative embodiment of our invention.

DETAILED DESCRIPTION STRUCTURE Turning now to the FIGURE, there is shown a partially cut away view of an illustrative embodiment of our invention comprising an evacuable chamber 10 including a sublimation pump 100 and, in gas flow communication with the chamber, a cryogenic roughing pump 200 and a cold trap 300. Hereinafter the term communicates" shall refer to gas flow communication.

The evacuable chamber l0 includes a cylindrical upper bell jar portion l2, the interior of which serves as a working area, and which communicates with the sublimation pump 100. The upper portion includes a port or demountable seal 14 which enables access to the interior of chamber 10 and further includes, on the exterior surface thereof, a plurality of heating elements 16 electrically connected to one another and to a suitable signal source (not shown) via terminals 18. The upper portion is also provided with a pair of apertures 20 and 22, one of which (20) is coupled to roughing pump 200 and the other of which (22) is coupled to cold trap 300.

Illustratively, the upper portion l2 is made from oneeighth inch thick stainless steel and the heating elements are commercially available electric heating tapes (resistance wire). The signal source typically provides about l200 watts of power over an eight hour period for outgassing a sixty liter chamber 10.

The sublimation pump 100 comprises a generally cylindrical lower portion 102 of chamber 100 which surrounds a source 104 of a sublimable getter material. Getter source 104, shown schematically, extends through the bottom of chamber l0 via a demountable seal 106 to a pair of terminals 108. Another signal source, not shown, is connectable to terminals 108 for providing sufficient power to heat getter source 104 above its sublimation temperature. Sublimation causes the deposition of a thin film of getter material on the walls of lower portion 102 which have the shape of a bellows. The latter shape increases the surface-tovolume ratio of the sublimation pump thereby enhancing its pumping speed for a given volume sublimation pump. Optionally, a plurality of baffles 110 are positioned between the upper and lower portions of chamber l0 in order to prevent sublimed getter material from being deposited in the working space or on samples disposed therein. In practice, the baffles 110 are arranged in overlapping relationship in order to block line-of-sight paths between the getter source 104 and the upper portion l2. Where conditions permit, however, the baffles may be omitted and/or the lower portion may serve as a working space. In the latter case, the heating elements 16 and the apertures 20 and 22 would be adapted to the lower portion 102. In this regard, the liquid nitrogen dewar 112, which surrounds lower portion 102, is made to be removable. Consequently, the heating of the chamber for outgassing and the cooling of the thin film for low pressure pumping are not inconsistent because, as described hereinafter, the two procedures take place at different times in the pumping cycle.

Alternatively, dewar 112 may be replaced with a lower temperature refrigerator such as the POLY- COLD (Model PCT 015) manufactured by Gulf Western Metals Forming Company, Danville, Illinois. This model is advertised as being capable of producing temperatures as low as about 133 degrees I( which is suitable for some applications of our invention described hereinafter. Although such a refrigerator may be more convenient than having to periodically refill dewar 112 with liquid nitrogen (relatively inexpensive), it does add substantially to the total cost of the system.

Illustratively, the bellows-shaped lower portion 102 comprises a structure which is more typically used as an expandable pipe joint commercially available from the Anaconda Copper Company, Waterbury, Connecticut. The unitary chamber l0 is formed by welding the lower portion 102 to the upper bell jar portion 12, the latter being commercially available from Firestone Steel Products Company, Akron, Ohio. To form an approximately liter chamber l0, the upper portion has a 12inch diameter and the lower portion, made from stainless steel 0.06 inches thick, has an inside diameter of l2 inches and an outside diameter of I4 inches. A suitable bellows has about one convolution per inch. To further increase the surface-to-volume ratio, however, commercially available bellows having more convolutions per inch may be used.

A suitable high quality getter source 104 is illustratively a titanium-molybdenum filament commercially available from Varian Associates, Palo Alto, California. These devices typically include a plurality of separately heatable filaments which are reusable a number of times.

The cryogenic roughing pump 200 comprises a bifurcated pipe having a main segment 202 and branch segments 204 and 206. The main segment 202 communicates with aperture 20 of chamber l0 through valve 205 mounted between demountable seals 208 and 210. Branch segment 206 communicates with a molecular sieve 212 through valve 214 mounted between demountable seals 216 and 218. Such sieves are commercially available from Ultek Corporation, Palo Alto, California. .Branch segment 204, on the other hand, forms what is known in the art as a .cold fingen that is, it

. gases which pass through branch segment 206 when valves 20S and 214 are open, pressed pellets 230 comprising sodium-aluminum-silicate are placed'in container 224.

The double dewar 222, which is also commercially available from Superior Air Products Company, Sayreville, New Jersey, comprises a first dewar 323 which contains the pool of liquid helium 221 into which the end of branch segment 204 is submersed, and a second dewar 234 which contains a pool of liquid nitrogen 236 into which the first dewar 232 is partially submersed. In order to increase the surface-to-volume ratio of the pumping portion of branch segment 204,l the end portion thereof is filled with a gas absorbing material such as sintered copper pellets 238.

i The cold trap 300 comprises a pipe having a main -segment302 which communicates with an aperture 22 in chamber and another cold finger, i.e., asingle branch segment 304 which is closed at its'end 306, the

latter being submersed in a pool of liquid helium ina liquid-nitrogen, liquid helium double dewar-222. The

latter is shown in phantom because in practice it may 4 be the same double dewar (i.e., dewar 222) which is used in the roughing pump200. As described hereinafter, the use of dewar 222 in both the roughing pump 200 and the cold trap 300 is possible because the two function at different times in the pumping cycle. Alternatively, of course, separate dewars (either single or double) may be used, or one skilled in the art might readily adapt one dewar to function for both the roughing pump and the cold trap without requiring that the dewar be physically moved from branch segment 204 to branch segment 304 (or vice versa).

Illustratively, pipe segments 202, 206 and 302 are one-sixteenth inch thick, l.5 inch diameter stainless steel whereas branch segments 204 and 304 are about 0.02 inches thick, one-half inch diameter stainless steel.. Similarly, the double dewars are made from 0.03 inch thick stainless steel and are a typical doublewalled commercially available variety having a vacuum space between the walls.

OPERATION Initially dewar 112, 222 and 222 are in a lowered position so that the bellows is not cooled by liquid nitrogen and the ends of branch segments 204 and 304 are not cooled by liquid helium. The raising and lowering of the dewars may be accomplished by hydraulic lifters or other suitable apparatus'not shown. The pumping-of chamber 10 then proceeds as follows:

l. a commerically available mechanical blower (not shown) is coupled to the chamber through a demountableseal (such as that shown at 14) in order to reduce the chamber pressure from one atmosphere to about 0.1 atmosphere. This step, however, is optional;

2. valves 205 and 214 are opened to enable the molecular sieve 212 to reduce the chamber pressure to about 104 Torr. This type of adsorption pump is effective in removing gases such as nitrogen, oxygen and water vapor;

3. valve 214 is closed and dewar 222 is raised so that the end of branch segment 204 is submersedin liquid helium.. Cooling of segment 204 to liquid helium temperatures (4.2 degrees K) causes gases to condense on the interior surface thereof. ln order to prevent satura'- tion of the relatively small cold trap thereby formed at theend of segment 204, sintered copper 238 is used'to increase itssurface-to-volume ratio; l

4. concurrent with step (3) the external electrical heating elements 16 are activated by applying an electrical signal to terminal 18. Forv the liter chamber described above a suitable signal delivers about 1200 watts of power. This step serves primarily to drive water vapor and hydrocarbons, which are condensed on the interior walls of chamber 10, into branch segment 204. After about eight hours of baking at'about 150 degrees C, the 60 liter chamber pressure is reduced lto about l0-6 Torr; Y,

6. dewar 1 12 is raised to cool the bellows to liquid nitrogen temperatures (77 degrees K) and getter source 104 (a titanium-molybdenum filament) is heated above its sublimation temperature by applying an electrical signal to terminals 108. A suitable signal provides 40 amperes of current at 7 volts AC for a time period of about one minute. Heating of the filament to about 1,200 degrees C causes a high purity, very thin (e.g., 50 Angstroms) titanium film to be deposited on the interiorsurface of the bellows. Alternatively, dewar 112 may be raised after the titanium film has been deposited. ln either case, however, the titanium film, which has a very active surface for adsorbing gases, is effective to reduce the chamber pressure to about l09 Torr depending on the amount of residual helium and neon in the chamber. The low chamber pressure at which the titanium film functions and the largesurface area provided by the bellows cooperate to prevent the film from saturating with gases and thereby importantly enable continuous pumping under working conditions.

Where saturation is a problem, however, a fresh titanium film can be evaporated to regenerate the surface area of the bellows.

An additional feature of our invention, as mentioned previously, residesin the discovery that the titanium filmwill pump light hydrocarbons, such as methane, to pressures of about l0-ll Torr provided that the film (i.e., the bellows) is maintained at a temperature belowv 4about 180 degrees K. Additionally, Ar will be pumped to similar pressures if the film temperature is below about 120 degrees K. Both of these conditions are satisfied by submersingthe bellows in liquid nitrogenat 77 y from 10-9 to l0"l Torr is only a matter of minutes.

Moreover, we have found that pressures of l,Il Torr can be repeatedly achieved within 24 hours after the system has been exposed to room'air. From a dynamic standpoint, our system is capable of maintaining pressures of l0NH Torr under gas load conditions with dynamic pumping speeds in the millisecond range.

It is to be Vunderstood that the above-described arrangements are merely illustrative of the many possible specific embodiments which can be devised to represent application of the principles of our invention. Numerous and varied other arrangements can be achieved in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

In particular, while the cryogenic roughing pump 200 is preferred from the standpoint of cost, effectiveness, providing a magnetic field-free environment and not contaminating the chamber with hydrocarbons, it is possible, where design or other considerations dictate, to achieve pressures of l01l Torr by using an ion pump or a diffusion pump instead. ln addition, while chamber l0 is illustratively shown to have two apertures 20 and 22 for coupling to the roughing pump 200 and the cold trap 300, respectively, it is possible for one skilled in the art to utilize a single aperture and appropriate values for alternately coupling to both. Finally, wherethe added expense can be tolerated, the dewars described herein may be replaced with appropriate refrigerators or refrigeration systems.

One more feature of our invention resides in its ability to purify materials. Thus, we have demonstrated that our system can be utilized to convert commercially available 3-9s pure (99.9 percent pure) niobium to 76-9s pure (99 .9 999 percent pure) nlitbigiwhich not commercially vavailable.Thismr-e'sult is achieved by placing impure niobium in the chamber, reducing the chamber pressure to about l0l1 Torr as previously described and then heating the impure niobium to a temperature near to, but below, its melting point (e.g., to about 2500 degrees C). lt is expected that the pumping performance of our invention will be enhanced by substituting such high quality niobium for the titanium getter source previously described. Thus, a niobium thin film deposited on the bellows would pump the chamber to lower pressures at faster dynamic pumping speeds.

What is claimed is:

l. An ultrahigh vacuum system comprising:

an evacuable chamber having a wall portion thereof 'in the shape of a bellows;

means for cooling said bellows to a cryogenic temperature;

a source of a sublimable getter material disposed in said chamber in a space at least partially enclosed by said bellows;

means for heating said getter material above its sublimation temperature to produce a thin film thereof on an interior surface of said bellows, said thin film being effective to adsorb gases, including light hydrocarbons, in said chamber and to reduce the pressure in said chamber to an intermediate level;

a source of liquid helium located exterior to said chamber; and

a first conduit having one end in gas flow communication with said chamber and the otherend thereof closed and submersible in said liquid helium, said other end when cooled by said liquid helium source being effective to remove residual helium and neon from said chamber and to further reduce the gas pressure in said chamber to at least 10" Torr.

2. The system of claim l wherein said source of liquid helium comprises a double dewar including a liquid helium dewar into which said closed end of said first conduit is insertable and a liquid nitrogen dewar in whic said liquid helium dewar is disposed.

3. The system of claim l wherein said getter material comprises a titanium-molybdenum alloy and said thin film comprises titanium.

4. The system of claim 3 wherein cooling means lowers the temperature of said bellows below about 180 degrees K, said thin film thereby being effective to pump light hydrocarbon gases from said chamber.

5. The system of claim 4 wherein said cooling means lowers the temperature of said bellows below about degrees K, said thin film thereby being effective to pump also argon gas from said chamber.

6. The system of claim 1 wherein said getter material comprises 6-9's pure niobium which is purified from lessfpurenriibiurn'by heating said less pure niobium while the pressure of said chamber is maintained below at least l0ll Torr by the cooperative pumping action of said roughing pump, said titanium film and said source of liquid helium, said cooling means lowers the temperature of said bellows belowabout K, said thin film thereby being effective to pump light hydrocarbon gases from said chambers.

7. The system of claim l wherein said chamber has an upper portion which serves as a working space and a lower portion in the shape of said bellows, and including baffle means disposed between said portions for blocking line-of-sight paths between said source of getter material and said upper portion.

8. The system of claim 7 wherein said means for heating said chamber includes a plurality of heating elements disposed in contact with the exterior of said upper portion.

9. The system of claim 7 wherein said means for cooling said bellows comprises a liquid nitrogen dewar into which said bellows is submersible.

l0. The system of claim l including a roughing pump, means for heating said chamber for a preselected time period, and first value means for initially placing said pump in gas fiow communication with said chamber when said chamber is heated for said preselected time period and for subsequently isolating said chamber from said pump after said time period.

ll. The system of claim 10 wherein said roughing pump includes:

a molecular sieve adsorption pump; -4

second valve means in gas flow communication with said adsorption pump;

a second conduit connecting said first and second valve means;

a third conduit having one end closed and its other end in gas flow communication with said second conduit; and

a source of liquid helium capable of being brought into contact with said one end during said preselected time period.

l2. The system of claim ll including a source of gas absorbing material disposed in said third conduit near to said closed end thereof.

comprises sintered copper. v l

14. The system of claim 12 wherein said molecular sieve adsorption pump comprises'a container in gas flow communication with said second valve, a molecular sieve source comprising sodium-aluminum-silicate disposed insaid container, and a liquid nitrogen dewar into which said container is submersed.

15. The system of claim 12 wherein said source of liquidhelium comprises a double dewar including a liquid helium dewar into which said third conduit is insertable and a liquid nitrogen dewar in which said liquid helium dewar is disposed.

16. An ultrahigh vacuum system capable of producing subatmospheric pressures of at least l-11 rI`orr and of maintaining such pressures under gas load conditions at high dynamic pumping speeds comprising:

an evacuablel chamber having an upper portion,

which serves as a working space, and a lower portion in the shape of a rigid bellows, which comprises part of a sublimation pump;

first means for heating said upper portion;

. a cryogenic roughing pump connectable in gas flow communication with said chamber for reducing the pressure thereof to a first pressure of about l06 Torr when said upper portion is heated;

irstvalve means for isolating said roughing pump from said chamber after said first pressure is atin said chamber and to reduce the pressure thereof;

rto a second pressureof about l0*9 Torr;

baffle means disposed between said upper and lower portions for blocking line-of-sight paths between said `titaniumsource and said upper portion but which permit gas flow 'communication between said portions; and

helium cold trap means connectable in gas flow comsure is attained for pumping helium and neon gas, said cold trap means being effective to reduce the pressure of said chamber to at least 10'1l Torr.

17. The system of claim 16 wherein said cold trap means comprises K a second dewar for carrying liquid helium, a second conduit having one end open and in gas flow communication with 'said chamber and the other end closed thereof and submersible in said second liquid helium dewar. v 18. The system of claim 16 wherein said first heating means comprises a plurality of electrical heating elements in contact with the exterior of said upper portion and wherein said upper portion includes a port for gaining access to said chamber.

. 19. The system of claim 16 wherein said cryogenic roughing pump comprises:

a molecular sieve adsorption pump connectable in gas flow communication with said chamber, a first dewar for carrying liquid helium,

.munication with said chamber after said second presl a first conduit having one end open and connectable in gas flow communication with said chamber andV the other end closed and submersible in said first liquid helium dewar, and

a source of sintered copper disposed in said other end of said first conduit for increasing the surface-tovolume ratio thereof.

20. The system of claim 19 wherein said adsorption pump comprises: y

a container connectable in gas flow communication with said chamber,

a source of sodium-aluminumasilicatev disposed in said container, and l a second liquid nitrogen dewar into which said container is submersible.

21. The system of claim 16 wherein said titanium source includes at least one filament comprising a titanium-molybdenumv alloy. y

22. The system of claim 16 wherein said cooling means lowers to the temperature of said thin film to at least l2() degrees K, said cooled film thereby being effective to adsorb argon gas as well as `methane gas.

23. The system of claim 22 wherein said cooling means comprises a first liquid nitrogen dewar into which said bellows is submersible.

24. A method of reducing the pressure of a chamber to at least l011 Torr and of maintaining that pressure under gas load conditions at relatively high dynamic pumping speeds, and wherein said chamber and'associated apparatusincludes: l

l; a'wall portion of said chamber in the shape of a rigid bellows; 2. a source of sublimable titanium disposed in said chamber in a space at least partially surrounded bysaid bellows; and

3. a first conduit having one end open and in gas flow communication with said chamber and the other end thereof closed;

said method comprising the steps of:

a; placing a roughing pump in gas flow communication with said chamber;

. b. Vwhile said roughing pump is in communication with said chamber, heating said chamber to 'reduce its pressure 'to a first level;

c. after said first level is reached, isolating said `roughing pump from said chamber;

d. heating said titanium source above its sublimation temperature to effect the deposition of a thin film of titanium on an interior surface of said bellows;

e. cooling said titanium film to a temperature of at least 180 degrees K, so that said thin film adsorbs 'at least methane gas from said chamber and thereby reduces the pressure of said chamber to approximately l0'9 Torr; and

f. cooling said other end of said first conduit to the temper-ature of liquid helium approximately, thereby to lower the pressure of said chamber to approximately l0-11 Torr.

25. The method of claim 24 wherein said thin film is cooled to the temperature of liquid nitrogen approximately.

26. The method of claim 24 wherein said cooling step (e) lowers the temperature of said thin film to at least l degrees K so that said thin film also adsorbs argon gas.

27. The method of claim 26 wherein said chamber and associated apparatus further includes:

4. a second conduit having one end connectable in gas flow communication with said chamber and the other end thereof closed', 5. gas absorbing material disposed in said second conduit near to said other end thereof; and 6. a molecular sieve adsorption pump connectable in gas flow communication with said chamber; and wherein step (a) of claim 26 includes the sub-steps of: al. placing said adsorption pump in gas flow communication with said chamber to reduce the pressure of said chamber to an initial level above said first level;

a2. after said initial level is reached, isolating said adsorption pump from said chamber; and

a3. during some period of said heating step (b), cooling said other end of said second conduit to the temperature of liquid helium approximately to reduce' the pressure of said chamber to said first level. y 28. The method of claim 24 including, after a chamber pressure of about 10" Torr is reached, the additional steps of:

g. placing a body of relatively impure niobium in said- UM'IED STATES PA'11;N'1 OFFICE CERTIFICATE OF CORRECTICN Patent No. ?'81i9l Dated May 21,- 197i invehtods) L. R. N. Dunkleberger and L. Y. L. Shen It is certifglxed` that error appears in the above-identifiedpatent 'andy that said Letters Patentlare hereby corrected as shown below:

ColmmA 2, -lineV 2, before "high" delete "a".

Column 5, line l?, change "323" to --232.

Columnv a 6, line lit, change l'terminal" to .terminals.

' Column 9, line 30, change "dispersed" to "disposed".

Signed and sealed this lst day of October 1974.

McCOY M. GIBSON JR. l C. MARSHALL DANN Attesting Officer Commissioner of Patents I'ORM lio-1050 (1Q-69) uscoMM-oc soave-Poo l 0.8. Vllllllll PIIIYMO OWICI IMI 0-0-884. 

2. The system of claim 1 wherein said source of liquid helium comprises a double dewar including a liquid helium dewar into which said closed end of said first conduit is insertable and a liquid nitrogen dewar in which said liquid helium dewar is disposed.
 2. a source of sublimable titanium disposed in said chamber in a space at least partially surrounded by said bellows; and
 3. a first conduit having one end open and in gas flow communication with said chamber and the other end thereof closed; said method comprising the steps of: a. placing a roughing pump in gas flow communication with said chamber; b. while said roughing pump is in communication with said chamber, heating said chamber to reduce its pressure to a first level; c. after said first level is reached, isolating said roughing pump from said chamber; d. heating said titanium source above its sublimation temperature to effect the deposition of a thin film of titanium on an interior surface of said bellows; e. cooling said titanium film to a temperature of at least 180 degrees K, so that said thin film adsorbs at least methane gas from said chamber and thereby reduces the pressure of said chamber to approximately 10 9 Torr; and f. cooling said other end of said first conduit to the temperature of liquid helium approximately, thereby to lower the pressure of said chamber to approximately 10 11 Torr.
 3. The system of claim 1 wherein said getter material comprises a titanium-molybdenum alloy and said thin film comprises titanium.
 4. The system of claim 3 wherein cooling means lowers the temperature of said bellows below about 180 degrees K, said thin film thereby being effective to pump light hydrocarbon gases from said chamber.
 4. a second conduit having one end connectable in gas flow communication with said chamber and the other end thereof closed;
 5. gas absorbing material disposed in said second conduit near to said other end thereof; and
 5. The system of claim 4 wherein said cooling means lowers the temperature of said bellows below about 120 degrees K, said thin film thereby being effective to pump also argon gas from said chamber.
 6. a molecular sieve adsorption pump connectable in gas flow communication with said chamber; and wherein step (a) of claim 26 includes the sub-steps of: a1. placing said adsorption pump in gas flow communication with said chamber to reduce the pressure of said chamber to an initial level above said first level; a2. after said initial level is reached, isolating said adsorption pump from said chamber; and a3. during some period of said heating step (b), cooling said other end of said second conduit to the temperature of liquid helium approximately to reduce the pressure of said chamber to said first level.
 6. The system of claim 1 wherein said getter material comprises 6-9''s pure niobium which is purified from less pure niobium by heating said less pure niobium while the pressure of said chamber is maintained below at least 10 11 Torr by the cooperative pumping action of said roughing pump, said titanium film and said source of liquid helium, said cooling means lowers the temperature of said bellows below about 180* K, said thin film thereby being effective to pump light hydrocarbon gases from said chambers.
 7. The system of claim 1 wherein said chamber has an upper poRtion which serves as a working space and a lower portion in the shape of said bellows, and including baffle means disposed between said portions for blocking line-of-sight paths between said source of getter material and said upper portion.
 8. The system of claim 7 wherein said means for heating said chamber includes a plurality of heating elements disposed in contact with the exterior of said upper portion.
 9. The system of claim 7 wherein said means for cooling said bellows comprises a liquid nitrogen dewar into which said bellows is submersible.
 10. The system of claim 1 including a roughing pump, means for heating said chamber for a preselected time period, and first value means for initially placing said pump in gas flow communication with said chamber when said chamber is heated for said preselected time period and for subsequently isolating said chamber from said pump after said time period.
 11. The system of claim 10 wherein said roughing pump includes: a molecular sieve adsorption pump; second valve means in gas flow communication with said adsorption pump; a second conduit connecting said first and second valve means; a third conduit having one end closed and its other end in gas flow communication with said second conduit; and a source of liquid helium capable of being brought into contact with said one end during said preselected time period.
 12. The system of claim 11 including a source of gas absorbing material disposed in said third conduit near to said closed end thereof.
 13. The system of claim 12 wherein said material comprises sintered copper.
 14. The system of claim 12 wherein said molecular sieve adsorption pump comprises a container in gas flow communication with said second valve, a molecular sieve source comprising sodium-aluminum-silicate disposed in said container, and a liquid nitrogen dewar into which said container is submersed.
 15. The system of claim 12 wherein said source of liquid helium comprises a double dewar including a liquid helium dewar into which said third conduit is insertable and a liquid nitrogen dewar in which said liquid helium dewar is disposed.
 16. An ultrahigh vacuum system capable of producing subatmospheric pressures of at least 10 11 Torr and of maintaining such pressures under gas load conditions at high dynamic pumping speeds comprising: an evacuable chamber having an upper portion, which serves as a working space, and a lower portion in the shape of a rigid bellows, which comprises part of a sublimation pump; first means for heating said upper portion; a cryogenic roughing pump connectable in gas flow communication with said chamber for reducing the pressure thereof to a first pressure of about 10 6 Torr when said upper portion is heated; first valve means for isolating said roughing pump from said chamber after said first pressure is attained; a titanium source dispersed in said chamber in a space at least partially surrounded by said lower portion; second means for heating said source above its sublimation temperature to produce a thin film of titanium on an interior surface of said bellows; means for cooling said thin film to a temperature of at least 180 degrees K; said cooled film thereby being effective to adsorb gases, including methane, in said chamber and to reduce the pressure thereof to a second pressure of about 10 9 Torr; baffle means disposed between said upper and lower portions for blocking line-of-sight paths between said titanium source and said upper portion but which permit gas flow communication between said portions; and helium cold trap means connectable in gas flow communication with said chamber after said second pressure is attained for pumping helium and neon gas, said cold trap means being effective to reduce the pressure of said chamber to at least 10 11 Torr.
 17. The system of claim 16 wherein said cold trap means comprisEs a second dewar for carrying liquid helium, a second conduit having one end open and in gas flow communication with said chamber and the other end closed thereof and submersible in said second liquid helium dewar.
 18. The system of claim 16 wherein said first heating means comprises a plurality of electrical heating elements in contact with the exterior of said upper portion and wherein said upper portion includes a port for gaining access to said chamber.
 19. The system of claim 16 wherein said cryogenic roughing pump comprises: a molecular sieve adsorption pump connectable in gas flow communication with said chamber, a first dewar for carrying liquid helium, a first conduit having one end open and connectable in gas flow communication with said chamber and the other end closed and submersible in said first liquid helium dewar, and a source of sintered copper disposed in said other end of said first conduit for increasing the surface-to-volume ratio thereof.
 20. The system of claim 19 wherein said adsorption pump comprises: a container connectable in gas flow communication with said chamber, a source of sodium-aluminum-silicate disposed in said container, and a second liquid nitrogen dewar into which said container is submersible.
 21. The system of claim 16 wherein said titanium source includes at least one filament comprising a titanium-molybdenum alloy.
 22. The system of claim 16 wherein said cooling means lowers to the temperature of said thin film to at least 120 degrees K, said cooled film thereby being effective to adsorb argon gas as well as methane gas.
 23. The system of claim 22 wherein said cooling means comprises a first liquid nitrogen dewar into which said bellows is submersible.
 24. A method of reducing the pressure of a chamber to at least 10 11 Torr and of maintaining that pressure under gas load conditions at relatively high dynamic pumping speeds, and wherein said chamber and associated apparatus includes:
 25. The method of claim 24 wherein said thin film is cooled to the temperature of liquid nitrogen approximately.
 26. The method of claim 24 wherein said cooling step (e) lowers the temperature of said thin film to at least 120 degrees K so that said thin film also adsorbs argon gas.
 27. The method of claim 26 wherein said chamber and associated apparatus further includes:
 28. The method of claim 24 including, after a chamber pressure of about 10 11 Torr is reached, the additional steps of: g. placing a body of relatively impure niobium in said chamber, and h. heating said body to a temperature effective to cause gases therein to exude therefrom so that said body becomes approximately 99.9999 percent pure niobium.
 29. The method of claim 28 wherein said impure body of niobium is heated to a temperature near to, but less than, its melting point.
 30. The method of claim 29 wherein said impure body of niobium is heated to a temperature of about 2,500 degrees C. 